Fix a writing mistake

A small mistake found in the line of ```The default error is "can't be empty"``` for ```:presence``` helper. ```empty``` word changed to ```blank```.

guides/source/active_record_validations.md

@@ -528,7 +528,7 @@ If you validate the presence of an object associated via a `has_one` or
 Since `false.blank?` is true, if you want to validate the presence of a boolean
 field you should use `validates :field_name, inclusion: { in: [true, false] }`.
 
-The default error message is _"can't be empty"_.
+The default error message is _"can't be blank"_.
 
 ### `absence`